During the accident of the Three Mile Island nuclear power plant, a condition of low coolant level in the reactor vessel and, as a result, inadequate core cooling existed and was not recognized for a long period of time.
It was immediately recognized that improved instrumentation systems, including reactor vessel liquid level sensors, must be developed and inserted into all light water reactors. The problem was to find a reliable and accurate sensor and a method to insert this sensor into existing reactors as well as building the sensor into new reactors.
A differential pressure system for detecting level in the reactor vessel had been attempted but was found to be unreliable and inaccurate.
A second employed method was to place level sensors in the reactor pressurizer (connected to the pressurized reactor vessel) and infer somewhat ambiguously that the reactor vessel had a solid pack of coolant. This was the method used at Three Mile Island. The pressurizer is a second vessel connected to the reactor vessel and is close to and above the reactor. Because of a temperature differential reversal, the pressurizer was full of water and the reactor vessel had a large steam bubble trapped in it. Because the pressurizer instruments reported a full condition the operator turned off emergency pumps that were installed to keep the reactor full of coolant. This ultimately led to the problem that occurred because while the reactor was dry, high temperature failures occurred in the fuel rods.
A third method suggested comprises heated thermocouples but these read only a wet/dry condition in the reactor vessel above the fuel rod assemblies, and do not detect or read coolant temperature. Moreover, because of inaccuracies inherent in thermocouples, it is necessary to apply excessive amounts of heat in order to assure a positive differential temperature under all reactor conditions. When the sensor is "dry", the excessive heat, if not otherwise controlled, will melt the heater to failure and indicate that the sensor is "wet" when in fact it is "dry" and the heater has failed.